In a typical horizontal machining center, a large, heavy slide is vertically-movable on a machine base, and the slide carries a cutting tool. A horizontal boring machine has similar elements, and also has a tool quill which may be extended in cantilever fashion from the slide. A versatile horizontal machining center, the T-BAR Model of Cincinnati Milacron Inc. includes an extendable tool quill. The quill carries a central spindle which may be selectively locked or rotated, and the spindle grasps a tool at its outboard end. The tool is used, for example, to bore deep holes or to do in-line boring throughout the walls of a casting. In some instances, the radial sweep of a rotating tool is preset, and the workpiece remains stationary. In still other applications, where large-diameter holes must be bored, the boring tool may remain stationary at a fixed radial distance from the hole centerline, and the workpiece may be rotated to effect the relative cutting speed. A typical slide drive combines a hydraulic or electric motor with a rotatable ballscrew, threadably engaged with a non-rotating preloaded ball nut on the slide. This type of machine frequently includes a counterforce cylinder, which is applied to the slide to assist the screw in overcoming friction and slide weight. When the electrical power to the machine is stopped, hydraulic systems are shutdown, and typically, conventional lines to the counterforce cylinder will leak fluid back to a fluid tank. Brakes are also frequently employed, to hold the screw from rotating during the cutting operation, and to resist "overhauling", i.e., backdriving rotation of the ballscrew due to slide weight.
It is known in fluid systems to connect an accumulator in a supply line to provide for necessary fluid force during periods when a fluid pump is at idle or stopped, during machine operations. However, the known applications of accumulators for relatively short periods of machine operation, (e.g., in the nature of an hour or less), do not provide for a long-term shutdown where a slide position must be held for approximately 48 hours.
It is highly desirable that a horizontal machine be capable of maintaining the cutting tool position relative the workpiece within a substantially accurate tolerance [e.g., 0.0007 inch (0.018mm)]. When such an accurate position is held, economical production occurs for the reasons that costly reset-up time is avoided, and, the chance of cutting errors on a partially finished, expensive workpiece is substantially reduced.